Genesis 6:1-4

1 Now it came about, when men began to multiply on the face of the land, and daughters were born to them,

2 that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose.

3 Then the Lord said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.”

4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them.  Those were the mighty men who were of old, men of renown.

Other Scripture References:

1 Peter 3:18—20

2 Peter 2:4—5

Jude 6—7

Numbers 13:33

Quotes:

“[T]he wickedness of man had reached such proportion that they had engaged themselves in demonic relationships.  Not only did they not seek God, not only did they not endeavor to know God, but they pursued demons.” (Dr. John MacArthur)